Integrated hydrological model of the Central Kalahari Basin – optimal tool for assessment of sustainability of groundwater resources:powerpoint

Groundwater resources replenishment dynamics evaluation is critical for management purposes, especially in arid and semi-arid regions where it accounts for the majority of potable water demand. That evaluation, nowadays is done using distributed, integrated hydrological models(IHMs). Such models, inherently account for surface-groundwater interactions, relying on realistic hydrogeological conceptual models(HCMs)and reliable input data involving also surface hydrological fluxes like rainfall and potential evapotranspiration. In this study, the productive Central Kalahari Basin (CKB, ~200000km2) transboundary(Botswana and Namibia)Karoo Aquifer System IHM, was developed, to define its groundwater resources replenishment dynamics. An efficient data integration method for developing HCM of the CKB, was undertaken, applying 3-D geological modelling based on RockWorks code, in iterative combination with ArcGIS. Six hydrostratigraphic units(Kalahari Sand Unit, Stormberg Basalt Aquitard, Lebung Aquifer, Inter-Karoo Aquitard, Ecca Aquifer and Ghanzi Aquifer)were identified, their heads and related flow system interdependencies evaluated and hydraulic properties defined. The daily spatio-temporally variable IHM driving forces(rainfall and potential evapotranspiration)were defined using remote sensing technique. A fully 3-D, six-layer CKB IHM was developed using MODFLOW-NWTwithUZF1 package, accounting for variably saturated flow. The transient CKB IHM was calibrated throughout 13.5 years, using borehole hydraulic heads as state variables. Results of the model showed dominant water balance role of subsurface evapotranspiration restricting gross recharge to only few mm/year and typically negative, yearly net recharge (median -1.5mmy-1), varying from -3.6 (2013) to +3.0 (2006) mmy-1with respective rainfalls 287 and 664 mmy-1, explaining water table decline within the simulated period. Primary determinants of spatio-temporal distribution of net recharge in the CKB were amount and temporal distribution of rain, unsaturated zone thickness and vegetation type/density. The calibrated CKBIHM represents optimal tool for further assessment of impact of climate change upon the replenishment dynamics of groundwater resources of the CKB

Hydrogeology Journal

Only current active IAh members can view the link

Hydrogeology Journal

Only current active IAh members can view the link

Use of remote sensing and long-term in-situ time-series data in an integrated hydrological model of the Central Kalahari Basin, Southern Africa

Distributed numerical models, considered as optimal tools for groundwater resources management, have always been constrained by availability of spatio-temporal input data. This problem is particularly distinct in arid and semi-arid developing countries, characterized by large spatio-temporal variability of water fluxes but scarce ground-based monitoring networks. That problem can be mitigated by remote sensing (RS) methods, which nowadays are applicable for modelling not only surface-water but also groundwater resources, through rapidly increasing applications of integrated hydrological models (IHMs). This study shows implementation of various RS products in the IHM of the Central Kalahari Basin (~200 Mm2) multi-layered aquifer system, characterized by semi-arid climate and thick unsaturated zone, both enhancing evapotranspiration. The MODFLOW-NWT model with UZF1 package, accounting for variably saturated flow, was set up and calibrated in transient conditions throughout 13.5 years using borehole hydraulic heads as state variables and RS-based daily rainfall and potential evapotranspiration as driving forces. Other RS input data included: digital-elevation-model, land-use/land-cover and soils datasets. The model characterized spatio-temporal water flux dynamics, providing 13-year (2002–2014) daily and annual water balances, thereby evaluating groundwater-resource dynamics and replenishment. The balances showed the dominant role of evapotranspiration in restricting gross recharge to only a few mm yr−1 and typically negative net recharge (median, −1.5 mm yr−1), varying from −3.6 (2013) to +3.0 (2006) mm yr−1 (rainfall of 287 and 664 mm yr−1 respectively) and implying systematic water-table decline. The rainfall, surface morphology, unsaturated zone thickness and vegetation type/density were primary determinants of the spatio-temporal net recharge distribution

Hydrogeological conceptual model of large and complex sedimentary aquifer systems – Central Kalahari Basin (Botswana)

Successful groundwater resources evaluation and management is nowadays typically undertaken using distributed numerical groundwater flow models. Such models largely rely on hydrogeological conceptual models. The conceptual models summarize hydrogeological knowledge of an area to be modelled and thereby providing a framework for numerical model design. In this study, an efficient data integration method for developing hydrogeological conceptual model of the large and hydrogeologically-complex, Central Kalahari Basin (CKB) aquifer system, was undertaken. In that process, suitability of 3-D geological modelling with RockWorks code in iterative combination with standard GIS (ArcGIS) was tested. As a result, six hydrostratigraphic units were identified, their heads and related flow system interdependencies evaluated and hydraulic properties attached. A characteristic feature of the CKB is a thick unsaturated Kalahari Sand Unit (KSU), that restricts the erratic recharge input to <1 mm yr −1 in the centre to about 5–10 mm yr −1 in the eastern fringe. The analysis of the spatial distribution of topological surfaces of the hydrostratigraphic units and hydraulic heads of the aquifers, allowed to identify three flow systems of the three aquifers, Lebung, Ecca and Ghanzi, all three having similar radially-concentric regional groundwater flow patterns directed towards discharge area of Makgadikgadi Pans. That pattern similarity is likely due to various hydraulic interconnections, direct or through aquitard leakages, and also due to the presence of the overlying unconfined, surficial KSU, hydraulically connected with all the three aquifers, redistributing recharge into them. The proposed 3-D geological modelling with RockWorks, turned to be vital and efficient in developing hydrogeological conceptual model of a large and complex multi-layered aquifer systems. Its strength is in simplicity of operation, in conjunctive, iterative use with other software such as standard GIS and in flexibility to interface with numerical groundwater model. As a result of conceptual modelling, fully 3-d, 6 layer numerical model, with shallow, variably-saturated, unconfined layer is finally recommended as a transition from conceptual into numerical model of the CKB

Validation of satellite-based rainfall in Kalahari

Water resources management in arid and semi-arid areas is hampered by insufficient rainfall data, typically obtained from sparsely distributed rain gauges. Satellite-based rainfall estimates (SREs) are alternative sources of such data in these areas. In this study, daily rainfall estimates from FEWS-RFE∼11 km, TRMM-3B42∼27 km, CMOPRH∼27 km and CMORPH∼8 km were evaluated against nine, daily rain gauge records in Central Kalahari Basin (CKB), over a five-year period, 01/01/2001-31/12/2005. The aims were to evaluate the daily rainfall detection capabilities of the four SRE algorithms, analyze the spatio-temporal variability of rainfall in the CKB and perform bias-correction of the four SREs. Evaluation methods included scatter plot analysis, descriptive statistics, categorical statistics and bias decomposition. The spatio-temporal variability of rainfall, was assessed using the SREs' mean annual rainfall, standard deviation, coefficient of variation and spatial correlation functions. Bias correction of the four SREs was conducted using a Time-Varying Space-Fixed bias-correction scheme. The results underlined the importance of validating daily SREs, as they had different rainfall detection capabilities in the CKB. The FEWS-RFE∼11 km performed best, providing better results of descriptive and categorical statistics than the other three SREs, although bias decomposition showed that all SREs underestimated rainfall. The analysis showed that the most reliable SREs performance analysis indicator were the frequency of “miss” rainfall events and the “miss-bias”, as they directly indicated SREs' sensitivity and bias of rainfall detection, respectively. The Time Varying and Space Fixed (TVSF) bias-correction scheme, improved some error measures but resulted in the reduction of the spatial correlation distance, thus increased, already high, spatial rainfall variability of all the four SREs. This study highlighted SREs as valuable source of daily rainfall data providing good spatio-temporal data coverage especially suitable for areas with limited rain gauges, such as the CKB, but also emphasized SREs' drawbacks, creating avenue for follow up research